Process and apparatus for producing laminated materials

ABSTRACT

A process for producing laminated materials in which one or more polymer films are laminated to a metal strip by a combination of heat and pressure. Generally, polymer films are applied to both sides of the metal strip in a lamination nip which exerts a force of at least 15 kN/m. The nip rolls may also be at different temperatures, either by one nip roll being of conducting material such as steel and the other nip roll being of insulating material, or by actively cooling one of the rolls. This enables films having different softening temperatures to be laminated without risk of micro-blistering or of pick-up on the rolls.

This invention relates to a process and apparatus for producing alaminated metal sheet or strip. In particular, it relates to a thermallamination process for laminating a polymer film to a metal strip.

Thermal lamination of polypropylene films to metal strip, or sheet, isknown from GB-1324952, for example, for imparting corrosion resistanceto the metal strip. Simultaneous lamination of composite polyester filmsto each major surface of a metal strip is taught in EP-0312303 andsimultaneous lamination of a polyester film to one side of a metal stripand a polyolefin or polyamide-containing film to the other side isdescribed in EP-0312302.

Simultaneous lamination is achieved by applying one or more polymerfilms to a metal strip at a first temperature or laminating temperatureT₁ which does not damage the outer surface of the polymer film(s) as themetal strip and polymer film(s) pass through a lamination nip. There is,however, a limit to this metal temperature, because excessive heat willcause the polymer film to stick to the laminating nip roll. This isknown as "pick-up".

Polyester films, in particular, suffer micro-blistering at productionlaminating speeds. Whilst these defects are not thought to affect theintegrity of the PET film, they detract from the appearance and thus thecommercial viability of the film. Such micro-blistering can be avoidedby increasing the lamination temperature but this may then lead tosticking, or pick-up, of the film on the laminating roll.

Pick-up is more pronounced for polypropylene film and will start tooccur above a laminating temperature T₁ of only 180° C., dependent onthe film gauge. When thin polypropylene is to be laminated to one sideof a metal sheet with polyester, such as PET, simultaneously laminatedto the other side of the metal sheet, pick-up of the polypropylene willoccur if the laminating temperature is only 180° C., butmicro-blistering of the PET film will start to occur at lowertemperatures than 180° C.

This tendency to form micro-blisters under the polymer film isparticularly pronounced when the metal sheet has high surface roughnessvalues and varies according to the type and gauge of film used. It isbelieved that blistering arises due to air trapped beneath a film,particularly when the metal sheet has high surface roughness.

There is therefore a need for a lamination process which does not leadto micro-blistering of a polymer film or to pick-up of a polymer film onthe laminating roll.

According to the present invention, there is provided a process forproducing a laminate of strip metal and a polymeric film comprising thesteps of: heating the metal strip to a first temperature T₁ above theinitial adhesion point of the polymeric film; applying the polymericfilm to the preheated metal by passing the metal and film between niprolls; reheating the metal laminate to a second temperature T₂ to causethe film to interact with and bond to the metal sheet; and quenching thelaminate rapidly and uniformly; in which the laminating nip rolls imposea nip force in excess of 15 kN/m.

Previous commercial lamination processes, such as those described inEP-0312302 and EP-0312303, have used a lamination nip force of up to12.5 kN/m. The use of higher nip forces as in the present invention isparticularly advantageous when the polymer film is of polyester,particularly polyester having pigment therein, since these forces havebeen found to reduce and/or eliminate micro-blistering under thepolyester film which arises when commercial lamination speeds of 30m/min and above are used.

Typically, polyester films of 15 μm thickness for clear polyester and 25μm thickness for pigmented polyester were used but blistering arose bothabove and below these values when commercial lamination speeds wereused.

The problem of microblistering has not been apparent before since lowerline speeds were used. Furthermore, nip forces were limited by theapparatus used, heavier duty equipment being both bulkier and moreexpensive.

Micro-blistering may be due to lack of intimate contact between a metalsheet and a polymer film and can be alleviated by the use of higher nipforces.

Preferably, the lamination nip comprises an insulating roll and aconducting roll. Usually, the insulating roll is rubber coated and theconducting roll is of metal, preferably steel.

The conducting roll may also be cooled, for example by air directed ontothe roll or by water which may pass directly or in a zig-zag mannerthrough the roll. Previously, both laminating nip rolls have been rubbercoated. As a result, when thin polypropylene films are laminated at atemperature T₁ in excess of 180° C. the film starts to pick up on thelaminating roll. Thin polypropylene films are generally less than about100 μm thick. The product quality of laminates which have been subjectto such pick-up is not acceptable.

When a conducting roll is used, particularly on the side of a metalsheet to be coated with polypropylene, when polypropylene film is used,it was surprisingly found that laminating temperatures of up to 220° C.were possible without pick-up.

Preferably, the laminating nip rolls impose a force of at least 25 kN/m.The nip, or lamination, force is usually 40 kN/m but forces of up to 125kN/m and above are also possible. This could be advantageous asproduction speeds are increased.

Preferably, the nip force is applied to both of the rolls by means ofpneumatics although hydraulics, for example, may be more advantageouswhen loads in excess of 40 kN/m are to be applied.

The first temperature T₁, or laminating temperature, may be up to 220°C.

In a preferred embodiment, there are two polymer films applied to themetal strip, one on each major surface, or side, of the metal strip.There may be a polyester film on one side of the metal strip and apolypropylene film on the other side, for example. It is preferable thata thermally conducting roll be used on the side of the polymer filmhaving the lower melting point. In this example, a conducting roll wouldbe used on the polypropylene side of the metal strip in the laminatingnip and a conventional rubber-coated roll on the polyester side.

In an alternative embodiment, cast polypropylene film may be used on oneside and oriented polypropylene film on the other side of the metalstrip. It has not been possible to use a combination of orientedpolypropyene and cast polypropylene before now since the castpolypropylene tends to stick to the laminating roll at temperatureswhich are necessary for lamination using oriented polypropylene. Thus inthis example, the conducting roll would be used on the castpolypropylene side of the metal strip and the insulating roll, orconventional rubber-coated, on the oriented polypropylene side.

According to a further aspect of the present invention, there isprovided an apparatus for producing a laminated material comprising apolymer film bonded to a metal substrate, said apparatus comprisingmeans for feeding a metal strip and a strip of polymer film to alamination nip, means for heating the metal strip to a first temperatureT₁ to cause initial adhesion of the polymer film to the metal strip, asensor for monitoring the temperature T₁, means for adjusting nip forceto at least 15 kN/m, means for reheating the resultant laminate to asecond temperature T₂ to cause the film to interact with and bond to themetal strip and means for quenching the resultant laminate rapidly anduniformly.

The adjusting means preferably comprises pneumatics or hydraulics butsimpler biasing means such as a spring in compression or tension may beused if precise loading levels are not required.

According to another aspect of the present invention, there is provideda process for producing a laminate of a first polymeric film, stripmetal and a second polymeric film comprising the steps of: heating themetal strip to a first temperature T₁ above the initial adhesion pointof the polymeric films; applying the polymeric films to the preheatedmetal by passing the metal and films between nip rolls, the first filmbeing on one side of the metal strip and the second film being on theother side of the metal strip; reheating the metal laminate to a secondtemperature T₂ to cause the films to interact with and bond to the metalsheet; and quenching the laminate rapidly and uniformly; and in whichthe first film has a lower softening point than the second film and thenip rolls are at different temperatures, the nip roll on the side of thefirst film being at a lower temperature than the nip roll on the side ofthe second film.

The cooler roll may be a conducting roll such as chromium coated steelor stainless steel and the warmer roll may be an insulating roll.Alternatively, either roll may be actively cooled by air or coolingliquid. The air may be "blasted" onto the roll or cooling liquid maypass through the roll either directly or in a zig-zag manner. It is thuspossible to laminate at a higher temperature than would normally bepossible for films having different softening points without pick-up ofone film occurring. Thus, for example, a high lamination temperature canbe used for a PET/metal/polypropylene laminate without pick-up of thepolypropylene or blistering of the PET.

In a still more preferred embodiment, the processes described above maybe used in conjunction with a heat/cool zone as described in ourcopending PCT patent application no. WO94/21456. This heat/cool zoneheats and/or cools the reheated laminate prior to quenching, whereby thelaminate enters the quenching stage at a substantially constanttemperature T₃, irrespective of line speed.

A preferred embodiment of process for producing a laminated metal stripwill now be described, by way of example only, with reference to thedrawings, in which: --

FIG. 1 shows an apparatus for laminating two polymer films to a metalsubstrate;

FIG. 2 is a cross section of an air cooling rig;

FIG. 3 is a graph of surface roughness against laminator nip roll forceusing rubber coated nip rolls;

FIG. 4 is a graph of surface roughness against lamination speed showingspeed-temperature interaction for 0.3×806 mm steel using one steel andone rubber-coated nip roll;

FIG. 5 is a like graph to FIG. 3 for 0.23×795 mm steel;

FIG. 6 is a like graph to FIGS. 3 and 4 for 0.17×795 mm steel;

FIG. 7 is a graph of gloss against lamination speed showingspeed-temperature interaction for 0.3×806 mm steel;

FIG. 8 is a like graph to FIG. 6 for 0.23×795 mm steel; and

FIG. 9 is a like graph to FIGS. 6 and 7 for 0.17×795 mm steel.

In FIG. 1 it can be seen that the apparatus comprises a first roll 10over which a steel strip 15 is passed and second and third rolls 20, 30,over each of which a polymeric film strip 25, 35 respectively, ispassed. Pinch rolls 40, 45 bring the steel strip 15 and polymeric filmstrips 25, 35 together in lamination nip 50 and quenching apparatus 60immerses the resultant laminate 70 in a copious flood of cooling liquidin accordance with EP-B-0319309.

A preheater 100 is located between roll 10 and pinch rolls 40, 45 andserves to preheat the steel strip 15 to a temperature T₁ above theinitial adhesion point of the film strips 25, 35 before laminating atthe pinch rolls 40, 45. A second heater 110 reheats the laminate 70 to atemperature T₂ higher than the temperature T₁ at which the steel entersthe nip rolls and higher than the melting point of the polymeric films25, 35.

Generally, the laminate will then enter a heat/cool zone (not shown)which heats and/or cools the laminte to ensure that it enters thequenching apparatus 60 at a substantially constant temperature T₃,irrespective of line speed (see our copending patent application).

The quenching apparatus 60 comprises a reservoir 65 for containing acooling liquid 75, a pump 80 to draw liquid from the reservoir 65, aheat exchanger 85 to cool liquid delivered by the pump and a trough 90which receives cooled liquid from the heat exchanger 85. The laminate 70passes through trough 90 and is entirely flooded edge to edge withcooled liquid.

The cooling liquid 75 travels with the laminate 70 on its coatedsurfaces from the trough 90 into the reservoir 65 so prolonging heatexchange to ambient. The laminate 70 passes around turn roll 95 andfinally between wiper rolls 96, 97 which wipe off the cooling liquid.

FIG. 2 shows an apparatus for cooling a rubber-coated nip roll by forcedair. Cool air is blown by a fan into a cooling rig 240 which partiallyencloses one of the rubber-coated nip rolls 260. Air from the fan isblown through perforated plates 250, 255 and distributed across thewidth of the cooling rig 240 and onto the surface of the nip roll 260adjacent to a polypropylene film 200. This enables the laminatingtemperature to be increased to avoid blistering of a PET film 210simultaneously laminated to the other side of steel substrate 220without causing the polypropylene film to adhere to the nip roll.

EXAMPLE 1

An original nip roll force of 12.5 kN/m exerted by rubber nip rolls wasincreased in an attempt to reduce blister levels on white PET whenlaminated to one side of a metal substrate in conjunction withlamination of a polypropylene film to the other side. Film surfaceroughness was used to quantify the extent of microblistering, ratherthan simply visual assessment.

With reference to FIG. 3, it can be seen that as the nip roll force wasincreased, there was a decrease in the level of blistering. This wasapparent both visually and as measured surface roughness of the whitePET film. Both the surface roughness and the range of these surfaceroughness values were reduced as the nip force was increased up to 40kN/m. Mean surface roughness (Ra) values decreased from 0.73 μm to 0.34μm. The appearance of the product was also improved.

Furthermore, the actual range of surface roughness values for each niproll force decreased from 0.61 to 0.23 μm so that a greater consistencyof laminate quality was attained for increased nip roll force.

Lamination speeds were also increased from 20 m/min to 40 m/min withoutloss of surface quality, i.e. occurrence of microblistering as measuredby surface roughness of the polymer film when higher nip forces wereapplied.

It was thought that increase in nip force might lead to an increase intemperature of the rubber-coated rolls. Surface temperature of the rollswas monitored at nip forces of 28 kN/m and 40 kN/m after 30 minutesrunning time but no increase in the temperature of the rubber coatingwas recorded.

EXAMPLE 2

A series of trials were carried out using one water-cooled steel rolland one rubber coated roll in the laminator. Pre-heat temperature, niproll force and line speed were adjusted to determine the optimumparameters for minimisation of blistering of PET films.

A laminate having clear polypropylene on one side of a steel substrateand white PET on the other side was manufactured. The steel nip roll wasfitted in the side of the laminator which was to receive thepolypropylene film.

In a first trial, steel strip having a width of 847 mm (commercialwidth) and 0.31 mm gauge was laminated with 830 mm wide white PET filmof 25 μm gauge and 826 mm wide clear polypropylene of 40 μm gauge. Watercooling was connected to flow through the steel laminating roll and linespeed increased to 32 m/min. The temperature of lamination T₁ could beincreased to 220° C.

A second trial was carried out whilst varying speed, nip roll force,steel gauge and temperature. Roughness of the white PET surface wasmeasured. Surface roughness and gloss were used as indicators ofblistering levels of the PET film. As the level of blistering increasedso did surface roughness, whilst the gloss level decreased. Details ofparameter interaction are shown in FIGS. 4 to 9, each trial beingconducted using one water-cooled steel roll and one rubber-coated roll.

At low speeds (8 m/min) there was no blistering, irrespective of thepre-heat exit temperature. Mean surface roughness was low and glosshigh. At increased speeds, blistering increased. The temperature T₁ (seeFIG. 1) was then raised from 165° C. to 200° C., which reducedblistering.

At line speeds of 8 m/min there was no need to raise the temperature T₁to avoid blistering but as line speed increased it was necessary toraise the temperature T₁ in order to reduce blistering. At speeds inexcess of 25 m/min, the temperature had to be approximately 200° C. toensure low levels of blistering.

Nip roll force did not have as significant an effect when using a steelnip roll as was observed when using two rubber coated rolls.

It was possible to use less expensive steel strip when using the processof the present invention. Stone finish tin free steel could be usedrather than the more expensive bright finish steel which was hithertorequired in order to reduce blistering. It is envisaged that inalternative embodiments different polymers may be used from thosedescribed in the specific examples and benefits additional to avoidanceof blistering can be achieved, such as the use of oriented polypropylenefilm with cast polypropylene film.

We claim:
 1. A process for producing a laminate of strip metal and apolymeric film comprising the steps of:heating the metal strip to afirst temperature T₁ above the initial adhesion point of the polymericfilm; applying the polymeric film to the preheated metal by passing themetal and film between nip rolls thereby forming a metal laminate;heating the metal laminate to a second temperature T₂ above the firsttemperature T₁ to cause the film to interact with and bond to the metalsheet; quenching the laminate rapidly and uniformly; and the laminatingnip rolls impose a nip force in excess of 15 kN/m.
 2. A processaccording to claim 1, in which the nip rolls impose a nip force of atleast 25 kN/m.
 3. A process according to claim 1 or claim 2, in which asecond polymeric film is laminated to the strip metal in whichtheheating of the metal strip to the first temperature T₁ is also abovethe initial adhesion point of the second polymeric film; the secondpolymeric film is applied to the preheated metal by passing the metaland both films between nip rolls, the first film being on one side ofthe metal strip and the second film being on the other side of the metalstrip; the heating of the metal laminate to the second temperature T₂also causes the second film to interact with and bond to the metalstrip; and the quenching step quenches both films of the laminaterapidly and uniformly; and the first film has a lower softening pointthan the second film, the nip rolls are at different temperatures, andthe nip roll on the side of the first film being at a lower temperaturethan the nip roll on the side of the second film.
 4. A process accordingto any one of claims 1 to 3, in which the nip rolls comprise aninsulating roll and a conducting roll.
 5. A process according to any oneof claims 1 to 3, in which one nip roll is actively cooled.
 6. Anapparatus for producing a laminated material comprising a polymer filmbonded to a metal substrate, said apparatus comprising:means for feedinga metal strip and a strip of polymer film to a lamination nip; means forheating the metal strip to a first temperature T₁ to cause initialadhesion of the polymer film to the metal strip thereby forming alaminate; a sensor for monitoring the temperature T₁ ; means for heatingthe resultant laminate to a second temperature T₂ above the firsttemperature T₁ to cause the film to interact with and bond to the metalstrip; means for quenching the resultant laminate rapidly and uniformly;and means for adjusting the nip force to at least 15 kN/m.
 7. A processfor producing a laminate of a first polymeric film, strip metal and asecond polymeric film comprising the steps of:heating the metal strip toa first temperature T₁ above the initial adhesion point of the polymericfilms; applying the polymeric films to the preheated metal by passingthe metal and films between nip rolls thereby forming a metal laminate,the first film being on one side of the metal strip and the second filmbeing on the other side of the metal strip; heating the metal laminateto a second temperature T₂ above the first temperature T₁ to cause thefilms to interact with and bond to the metal sheet; quenching thelaminate rapidly and uniformly; the first film has a lower softeningpoint than the second film, the nip rolls are at different temperatures,and the nip roll on the side of the first film being at a lowertemperature than the nip roll on the side of the second film.
 8. Aprocess according to claim 7, in which the cooler roll is a conductingroll and the warmer roll is an insulating roll.
 9. A process accordingto claim 7, in which the cooler roll is actively cooled.
 10. A processaccording to claims 1, 2, 7, 8 or 9, further comprising heating and/orcooling the heated laminate prior to quenching, whereby the laminateenters the quenching stage at a substantially constant temperature T₃,irrespective of line speed.